Fix VideoStreamEncoderTest.QualityScalerTriggersOnFrameDropping on iOS

https://webrtc-review.git.corp.google.com/c/src/+/447001 introduced a
test case which verifies plumbing of encoder frame drop signals all
the way down to the quality scaler. Turns out that on iOS that feature
isn't used so the test will fail - and no currently enabled test bot
exercised that code path.

This CL checks for the existence of that feature and skips the test if
it is absent. Verified by running with a hacked local
`#define WEBRTC_IOS` and by force-including an extra try bot.

Bug: webrtc:467444018
Change-Id: I38c28a16553bcc22f1c907a24f0399322ede8d3d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/448340
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46878}